Subpatterns

Subpatterns are delimited by parentheses (round brackets),
which can be nested. Marking part of a pattern as a subpattern
does two things:

It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|) matches one of the words "cat",
"cataract", or "caterpillar". Without the parentheses, it would match
"cataract", "erpillar" or the empty string.

It sets up the subpattern as a capturing subpattern (as defined above).
When the whole pattern matches, that portion of the subject string
that matched the subpattern is passed back to the caller via the
ovector argument of pcre_exec().
Opening parentheses are counted from left to right (starting from 1) to
obtain the numbers of the capturing subpatterns.

For example, if the string "the red king" is matched against
the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king",
and are numbered 1, 2, and 3.

The fact that plain parentheses fulfill two functions is not
always helpful. There are often times when a grouping subpattern
is required without a capturing requirement. If an
opening parenthesis is followed by "?:", the subpattern does
not do any capturing, and is not counted when computing the
number of any subsequent capturing subpatterns. For example,
if the string "the white queen" is matched against the
pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and
are numbered 1 and 2. The maximum number of captured substrings
is 65535.

As a convenient shorthand, if any option settings are
required at the start of a non-capturing subpattern, the
option letters may appear between the "?" and the ":". Thus
the two patterns

(?i:saturday|sunday)
(?:(?i)saturday|sunday)

match exactly the same set of strings. Because alternative
branches are tried from left to right, and options are not
reset until the end of the subpattern is reached, an option
setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".

It is possible to name a subpattern using the syntax
(?P<name>pattern). This subpattern will then
be indexed in the matches array by its normal numeric position and
also by name. PHP 5.2.2 introduced two alternative syntaxes
(?<name>pattern) and (?'name'pattern).

Sometimes it is necessary to have multiple matching, but alternating
subgroups in a regular expression. Normally, each of these would be given
their own backreference number even though only one of them would ever
possibly match. To overcome this, the (?| syntax allows
having duplicate numbers. Consider the following regex matched against the
string Sunday:

(?:(Sat)ur|(Sun))day

Here Sun is stored in backreference 2, while
backreference 1 is empty. Matching yields Sat in
backreference 1 while backreference 2 does not exist. Changing the pattern
to use the (?| fixes this problem:

(?|(Sat)ur|(Sun))day

Using this pattern, both Sun and Sat
would be stored in backreference 1.